Use it or Lose it
The concept today is a very simple one.
Use it or lose it.
You've probably heard it many times, but have you internalized it? In Nassim Taleb's Antifragile he discusses the integral difference between organic (living) and inorganic (non-living) matter. Living things tend to overcompensate as a response to environmental stresses while non-living things degrade over time.
Thus, for non-living things, it's "Use it and lose it". Cars and washing machines depreciate every time you use them, while our bodies improve as a result of training.
The concept of antifragility refers to things that improve under little stressors and benefit from random events and complexity.
Keep the little stresses on, allow yourself to recover properly, and you too can become antifragile.